Hollow fiber membrane devices for fluids processing are generally assembled in a fashion resembling a shell-and-tube heat exchanger. A plurality of hollow fibers are arranged in an elongated bundle and encased in a shell containment housing. The hollow fibers are embedded in a tube sheet at both ends, which tube sheet is normally fabricated from a resin material. The fluid processing devices using hollow fiber membranes may be configured either as a shell side feed device or a bore side feed device.
In the shell side feed configuration, the fluid to be treated is introduced to the outside of the hollow fibers, and a portion of the fluid permeates through the fiber wall into the lumens of the hollow fibers. The permeate fluid is removed from one or both ends of the fiber lumens. The non-permeate fluid is discharged from a region on the shell side that is often longitudinally distant from the fluid inlet to achieve a desired co-current or countercurrent flow configuration. Clearly, in the shell side feed configuration, at least one tube sheet should be made in such a fashion that the bores of the hollow fibers communicate through the tube sheet and are open to the exterior face of the tube sheet that is opposite the fiber bundle. Such a tubesheet is referred to as an active tube sheet, which allows for introduction of a fluid to or removal from the fiber lumens. Depending on whether one or both tube sheets are active, the fiber bundle is considered to be single open-ended or double open-ended. Most commercial hollow fiber membrane devices for fluid separation adapt the shell side feed configuration.
Alternatively, in a bore side feed separator, the fluid to be treated can be admitted to the fiber lumens at one end of the hollow fiber device, and a non-permeate fluid exits from the fiber bores at the other end of the device. This requires two active tube sheets, one at each end of the fiber bundle. This latter configuration has been used for certain applications such as nitrogen production from air.
In fluid processing by membranes, a significant pressure differential across the membrane is often required to provide the driving force for mass transfer from one side of the membrane to the other side of the membrane. Compared to shell side feed, a more even flow distribution of feed on membrane surface is achieved in the bore side feed hollow fiber devices, which is beneficial to an efficient operation. Further, in the bore side feed configuration, only the fiber wall and the end caps are pressurized, and the pressure at the shell side of the membrane device is substantially low, which reduces the mechanical strength requirement of the casing shell of the device. However, when a pressurized fluid moves to or from the fiber bores, significant pressure is exerted on the tube sheets. Because there are no balancing forces on the inner surface of the tube sheets, the pressure causes the fiber bundle to collapse between the tube sheets.
Various prior art devices have support means for the fiber bundle and the tube sheets. U.S. Pat. No. 4,961,760 issued to Caskey et al. teaches a boreside feed hollow fiber membrane fluid separation device which has a cylindrical tube sheet support means which encases the hollow fiber bundle and embeds in the tube sheets at either end of the bundle. The tube sheet support means must be integrated into the hollow fiber bundle at the time of manufacture, adding to the expense and complexity of the hollow fiber bundles.
Alternatively, there are devices in which the casing itself acts as the tubesheet support means by having a cross-sectional diameter less than the diameter of the tubesheet. However, in these devices, at least one tube sheet must be formed in place after the hollow fibre bundle has been inserted into the casing which is a cumbersome procedure. In U.S. Pat. No. 4,929,259, a device is disclosed which requires formation of resin plugs to augment the tube sheets after they have been assembled with the casing shell.